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TPS56221: Questions about the example circuit in datasheet

Kevin Wu39
Prodigy 165 points
Part Number: TPS56221

Hi all, 

I'm trying to use the TPS56221 to create a 12V in, 5V 18A output application, but I'm getting a bit confused looking at the example provided in the datasheet. www.ti.com/.../tps56221.pdf

1. Why in section 8.2.2 R1 is listed as 1.78kOhm, but then in 8.2.2.11 it is listed as 2.83 kOhm ?

2. In Table 1 in section 8.2.1 what is Itran ? Is it always 10A or is that up to the user?

3. In Table 1 in section 8.2.1 Iout(max) is 25A, but then in  section 8.2.2.4 equation 10 uses Iout(max) = 30A (equation 9 uses 25A...), and then again in section 8.2.2.11 Iout(max) = 30A, where is this 30A coming from and why are they not using 25A throughout their calculations??

Thanks in advance!

  • 0
    TI__Expert 4365 points

    1. The resistor value 2.87 kOhm is the correct value and is shown in agreement in the EVM manual that corresponds to the design example. The value of 1.78 kOhm was probably from an early revision of the EVM and the updated value did not make it into the data sheet.
    2. The Itran value is up to the user. In this example they happen to use a 10A load step transient and the spec is to have less than 100mV output overshoot or undershoot.
    3. The rated output of the design example is 25A. But the current limit during short circuit must be higher than 25. This leads to the inductor saturation current rating that must not be exceeded as shown in Table 3.

  • 0 in reply to Alan Martin
    Prodigy 165 points

    Thanks! I have a few more questions

    1. Why are they using 30A if they originally set the Iscp (table 1 section 8.2.1) to 32A ?
    2. Does tss (softstart time, table 1) affect the Itrans - I thought that if we use soft start, we eliminate the 'step'

    EDIT: 

    3. Why do they use 30A when calculating Rocset in 8.2.2.11? Shouldn't they use 25A since 30A is for short circuit conditions?

  • 0 in reply to Kevin Wu39
    TI__Guru**** 191445 points
    There appears to be a little sloppy editing in that part of the datasheet. For the purposes of the example, it does not really matter whether it is 30 or 32 A. You can still use the equations to calculate the correct value for your design. You typically do want to set the over current set point higher than your actual maximum load current.

    SS time does not affect the Itrans value. What they are specifying there is the effect on the output voltage due to a sudden load step change. You can see an example of this effect in Figure 10 of the EVM users guide:
    www.ti.com/.../slvu446a.pdf
  • 0 in reply to JohnTucker
    Prodigy 165 points
    Ah I see! Thank you so much!!
  • 0 in reply to Kevin Wu39
    TI__Expert 4365 points

    I think 30A is the correct number in 8.2.2.11.  Rocset is for over-current, same meaning as short-circuit.